The present invention relates to rivets and, more particularly, to a so-called blind rivet especially adapted for the riveting of side-by-side workpieces where access is convenient from only one side of the workpieces. The side of the work from which the rivet is applied is referred to as the front or "access" side and will be so referred to herein. The other side is called the rear or "blind" side.
A blind rivet of the general type to which the present invention relates is described in U.S. Pat. No. 3,285,121 wherein it will be seen that a headed sleeve is telescoped over, and positioned on, an elongated and specially formed setting pin having a pulling section projecting beyond the headed end of the sleeve and an expanding and locking head projecting beyond the opposite end of the sleeve. This head has an expanding shoulder adjacent the end of the sleeve and a taper which progressively increases the size of the head beyond the expanding shoulder.
With its two basic parts in telescoped relation, the rivet customarily is inserted into aligned holes in two workpieces that are to be riveted, from the access side thereof, and is positioned with the head of the sleeve adjacent this side of the work and with the expanding head of the setting pin on the blind side. Then the pin is pulled from the access side while the sleeve is held against movement. This draws the expanding head into the blind-side end of the sleeve to clinch the workpieces together, expands the sleeve to fill the holes in the workpieces, and form a bulged, bulb-shaped head on the blind-side end of the sleeve.
As the setting operation is completed, a locking crown disposed around the setting pin within the sleeve head on the access side is swagged into a locking groove in the pin, and the pulling section of the pin is broken off substantially flush with the sleeve head. In this manner, the workpieces are clinched together and are held between the two heads on the opposite ends of the sleeve.
The tightness and fatigue life of each joint depend upon both the tensile and shear strengths of the rivet and the amount of bearing area that can be developed between the expanded, blind-side head of the blind-side workpiece without splitting or tearing the latter. Tensile and shear strengths, of course, depend primarily upon the strength of the materials used in the rivet, and increase as the hardness of the materials is increased. On the other hand, the amount of blind-side bearing area that can be developed depends upon the deformability of the sleeve, and the ductility of the expanding head, that is, the ability of the expanding head to wire-draw during the setting operation. This wire-drawing ability is particularly important to accommodate variations in the overall thickness of the work, or grip lenght, within permissible tolerances, and to avoid danger of splitting or tearing of the blind-side end of the sleeve or the blind-side workpiece, which sometimes has relatively low bearing strength.
The aforesaid patent discloses a rivet in which the material used represents a compromise between high strength and high ductility. The aforesaid application discloses an improved rivet in which the expanding head is formed with a central core of high strength and has a softer, more ductile peripheral layer of annealed metal which is readily deformable during setting of the rivet. Thus, the core imparts high strength, while the peripheral layer provides the desired ductility.
While it is possible to obtain such improved strength and head-forming characteristics by peripherally annealing the head of the setting pin, a need also has existed for a rivet with similar characteristics obtained without peripheral annealing, either because such annealing is impractical with certain materials, or because of the added manufacturing expense involved in the annealing operation. For example, it is desirable to use aluminum rivets in aircraft structures because aluminum is relatively strong and light in weight, but aluminum does not respond to annealing as rapidly as steel, and thus is difficult to anneal in the manner described in the aforesaid patent.